(1) Field of the Invention
This invention relates generally to the area of reading out electronic sensors, and more particular, to sensors having variable resistance as e.g. potentiometers.
(2) Description of the Prior Art
One of the most convenient and inexpensive transducer is a potentiometer. A potentiometer is essentially a device, which can be configured so that a continuously variable voltage is produced that is proportional to the potentiometer's shaft position.
One of the applications of potentiometers is the usage as an analog input device in combination with devices as a joystick. A joystick position is determined by reading the resistance value of a potentiometer attached to the joystick. Potentiometers are a simple and very inexpensive system and can be designed to interface to a standard computer interface. Potentiometers can use a desktop as a position reference, and a joystick is usually suction or non-skid mounted to the desktop to prevent sliding.
Two potentiometers are required to define a position in a two-dimensional space. FIG. 1 prior art shows a typical implementation of a circuit having a sensor 10 comprising two potentiometers 1 and 2, a shunt resistor 3, a supply voltage Vdd, a port 4 to measure the voltage representing the total resistance of the two potentiometers 1 and 2 and a port 5 to measure the voltages representing the resistance values of said potentiometers 1 and 2. The potentiometers 1 and 2 are acting as a voltage divider. The voltage Vs 6 is the entry voltage of the potentiometers 1 and 2. In order to get the resistance of each potentiometer the sum of the resistance Z of both the potentiometers 1 and 2 must be known:Z=R1+R2,wherein R1 is the variable resistance of potentiometer 1 and R2 is the variable resistance of potentiometer 2. The value of Z is measured by opening the switches 7 and 9 and closing switch 8. The total resistance of both potentiometers 1 and 2 calculates   Z  =            Vs      ×              R        3                            V        dd            -      Vs      wherein Vs is the voltage measured at port 4 and R3 is the known constant resistance of the resistor 3.
An additional switch 9 is required to measure the resistance of the potentiometers 1 and 2. For this measurement switch 7 is closed to have the complete voltage Vdd available for the maximum signal. Therefore Vs equals Vdd while switch 7 is closed.
The voltage V1 comprises the difference between the entry voltages of the sensor Vs 6 (in this case Vs equals Vdd) to the voltage measured at the output port 5.             V      1        =                            R1          Z                ×        Vs        ⁢                                   ⁢        or        ⁢                                   ⁢                  R          1                    =                                    V            1                    ×          Z                Vs              ,wherein R1 is the resistance of the potentiometer 1.
The voltage V2 comprises the voltage level measured at the output port 5.             V      2        =                                        R            2                    Z                ×        Vs        ⁢                                   ⁢        or        ⁢                                   ⁢                  R          2                    =                                    V            2                    ×          Z                Vs              ,wherein R2 is the resistance of the potentiometer 2.
There are various problems linked to this prior art approach. There is no continuous measurement of the resistance of R1, R2 and of the total resistance Z possible. There are two time slots required to measure first the total resistance Z and secondly the resistance of each potentiometer. For a following analog-to-digital converter the time slots available are very short to convert the voltage values measured at port 5 and at port 4 and that will increase the noise to signal ratio. The voltage modulation of the voltages measured at port 4 and 5 is huge and therefore the time constant of a filter is quite limited.
Additionally for the measurement of the total resistance Z the signals are quite small because the voltage has to be divided between the sensor 10 and the resistor 3.
U.S. Pat. No. (4,864,513 to Levine et al.) describes a technique for forming a signal representing the operator adjustment of a potentiometer. This technique consists of measuring the reference time constant of the resistance between the fixed terminals of the potentiometer and a capacitor, measuring the variable time constant of the resistance of the potentiometer between a fixed terminal and the terminal variable by an operator setting and this same capacitor, and again measuring the reference time constant of the resistance between the fixed terminals of the potentiometer and the capacitor. These time constant measurements are made by discharging the capacitor, charging the capacitor through the appropriate terminal of the potentiometer and measuring the time required for the voltage across the capacitor to reach a predetermined threshold level.
U.S. Pat. No. (5,247,680 to Huber) discloses a circuit including an adjustment control (e.g. a potentiometer) that has two end connections and a slider that is adjustable for setting different resistance ratios. The two end connections of the potentiometer are each connected to one of two connections of a microprocessor and the slider is connected to one connection of a capacitor. The other capacitor connection is connected to a predetermined potential. To each of the two microprocessor connections at least one controllable switch is connected for optionally connecting a first potential or a second potential to the relevant microprocessor connection. The microprocessor executes a program run in which the capacitor that has previously been brought into an initial charge state is recharged first via one resistance section and then via the other resistance section of the potentiometer. In each case a measurement value Z corresponding to a capacitor-recharging period is determined and from the determined measurement values Z a digital data word signal is generated by forming the quotient thereof. This digital signal represents the resistance ratio set by the slider of the potentiometer.
U.S. Pat. No. (5,786,808 to Khoury) describes a digital positioning system for a joystick. The system uses a potentiometer having one input coupled to a constant supply voltage and a second input coupled to a joystick game port for generating a variable resistance representative of a current position of the joystick. A constant current source is coupled to the joystick game port for generating a current for converting the variable resistance representative of the current position of the joystick to a voltage level representative of the current position of the joystick. An analog-to-digital converter circuit is coupled to the joystick game port and is used to convert the voltage level representative of the current position of the joystick to a digital representation of the current position of the joystick.